Heat storage device

ABSTRACT

A heat storage device comprises a heat storage tank  2  charged with a heat storage material  1  for storing the heat supplied from the outside, and a heat exchanger  3  for executing an injection and an extraction of heat between the inside of the storage tank  2  and the outside by the heat exchange between the heat storage material and a heat transfer medium. The heat exchanger  3  is disposed so as to execute a heat exchange between the central portion  2   a  in the heat storage tank  2  and the outside, and suppresses the natural convection of the heat storage material  1  of the outer portion  2   b  by, for example, dispersing a liquid-absorbent material  5  in the outer portion  2   b  surrounding the central portion in the heat storage tank  2 , whereby reduces the influence of external environment on the central portion  2   a  in the heat storage tank, thereby suppressing the heat loss toward the outside.

FIELD OF THE INVENTION

[0001] The present invention relates to a heat storage device fortemporarily storing heat.

DESCRIPTION OF THE RELATED ART

[0002] Basic constructions of conventional heat storage devices areillustrated in FIGS. 11 through 13 in the form of sectional views.

[0003] A heat storage device shown in FIG. 11 comprises a heat storagetank 32 charged with a heat storage material 31 such as water, and aheat exchanger 33 disposed in the heat storage tank 32, for executing aheat exchange with the outside. In order to store the heat of, forexample, a temperature T_(x) within this heat storage device, thetemperature of the heat storage material 31 is raised to T_(x) by addingthe heat of a temperature of at least T_(x) to the heat exchanger 33from the outside. When heat is needed at the outside, the heat of thetemperature T_(x) is extracted from the heat storage material 31 to theoutside, using the heat exchanger 33.

[0004] A heat storage device as shown in FIG. 12 comprises heat storagetanks 42 a and 42 b charged with a heat transfer medium 47 such as waterwhich also serves as a heat storage material for storing heat, whereinthe heat storage tank 42 a has a communicating tube 44 a for connectingthe heat storage tank 1 with the outside, wherein the heat storage tanks42 a and 42 b are connected by a communicating tube 44 b, and whereinthe heat storage tank 42 b has a communicating tube 44 c for connectingthe heat storage tank 42 b with the outside.

[0005] In the operation of this heat storage device, in order to storethe heat of, for example, a temperature T_(x), the heat transfer medium47 is sucked out from the communicating tube 44 c, and the temperatureof the heat transfer medium 47 is raised to at least T_(x) by addingheat to the heat transfer medium 47 at the outside, then the heattransfer medium 47 being injected from the communicating tube 44 a tothe heat storage tanks 42 a. The heat transfer medium 47 removed fromthe heat storage tank 42 a by this injection, arrives at the heatstorage tanks 42 a passing through the communicating tube 44 a. After awhile, the heat transfer medium 47 is sucked out from the communicatingtube 44 c, and executes a heat transport between the heat storage tank42 and the outside, then returning to the heat storage tank 42 a againpassing through the communicating tube 44 a . Thus, a circulation of theheat transfer medium 47 is accomplished. If heat is needed at theoutside, a process in the opposite direction to the injection process ofheat is executed. That is, the heat transfer medium 47 is sucked outfrom the communicating tube 44 a, and heat is absorbed and utilized atthe outside, and the heat transfer medium 47 of which temperature hasdecreased as a result of the absorption and utilization of heat, isreturned from the communicating tube 44 c to the heat storage tank 41 b.Due to this returning, the heat transfer medium 47 removed from the heatstorage tanks 42 b flows into the heat storage tank 42 a passing throughthe communicating tube 44 b. Thus, a circulation of the heat transfermedium 47 is accomplished.

[0006] An object of suitably dividing the heat storage tank is tosuppress the occurrence of a dead water region where flow is apt tostagnate within the heat storage tank, and to allow the circulation ofthe heat transfer medium 47 in injecting and extracting heat to beuniformly conducted by using the whole of the heat storage tank.

[0007] If the heat of a lower temperature than an ambient temperature isstored, the directions of flow of the heat transfer medium 47 in theprocesses of injection and extraction will be opposite to each other.However, the basic operation is similar to the foregoing.

[0008] On the other hand, in a heat storage device shown in FIG. 13, aheat storage material 51 utilizing mainly transition heat is chargedinto small vessels 56, which are accommodated in the heat storage tank52, and the heat storage tank 52 is provided with communicating tubes 55a and 55 b for making the heat storage tank 52 communicate with theoutside. By the heat transfer medium 57 which circulates through thesecommunicating tubes 55 a and 55 b and the heat storage tank, heatexchange means for executing a heat exchange with the outside isconstituted. In this case, the storage of heat is mainly executed by theheat storage material 51 which utilizes a transition heat. Although theheat transfer medium 57 functions as heat transport means, it does notmean that the heat transfer medium 57 does not participate in thestorage of heat.

[0009] In this heat storage device, in order to store the heat of, forexample, a temperature T_(x), the heat transfer medium 57 having atemperature of at least T_(x) is injected from the outside to the heatstorage tank 52 through the communicating tube 55 a. The heat storagematerial 51 is heated by the heat which has been released by the heattransfer medium 57 via the walls of the small vessels 56. The heattransfer medium 57 injected returns to the outside passing through thecommunicating tube 55 b, and is again heated to a temperature of atlease T_(x). Thus, a similar circulation is repeated. If heat is neededat the outside, a process in the opposite direction to the process ofheat injection is executed. That is, the heat transfer medium 57 isinjected from the outside to the heat storage tank 52 through thecommunicating tube 55 b. The heat transfer medium 57 injected is heatedby the heat released by the heat transfer medium 51, via the walls ofthe small vessels 56. The heat transfer medium 57 heated returns to theoutside passing through the communicating tube 55 a, and its temperaturebecomes lower than T_(x) as a result of the utilization at the outside,then being injected again. Thus, a similar circulation is repeated. Alsoin the case where the heat of a lower temperature than the ambienttemperature is stored, the basic operation is similar to the foregoing.

[0010] In this example of constitution, since the heat transfer medium57 which is injected and extracted from the outside passes through thegaps between the small vessels, the area of a heat transfer surface tothe heat storage material 51 becomes large, as well as the treatment ofthe heat storage material 51 becomes easy, and hence such a constitutionis often used for heat storage devices that use paraffin as a heatstorage material 51 which utilizes transition heat.

[0011] In the above-described heat storage devices shown in FIGS. 11through 13, in the state of heat storage, if there are temperaturedifferences between the heat storage materials 31, 51, and the heattransfer medium 47 which also serves as a heat storage material, and theexternal environment surrounding the heat storage tanks 32, 42, and 52,then heat transfer always takes place between the heat storage devicesand the external environment through the wall surfaces of heat storagetanks 32, 42, and 52, respectively.

[0012] Neglecting the influence of heat radiation which is usually low,the heat loss Q of a heat storage material or a heat transfer mediumwhich also serves as a heat storage material (hereinafter these are bothreferred to as a heat storage material) is expressed by the followingequation.

Q=∫ ₀ ^(t) kA(Tx′−T ₀)dt [J]  (1)

[0013] Here, k denotes an overall heat transfer coefficient determinedby the material, construction, and ambient air speed of a heat storagetank, etc. A denotes a contact area between a heat storage tank and aambient fluid (such as air). T₀ represents an environmental temperatureoutside the heat storage device, and T_(x)′ represents a temperature atthe surface where the heat storage material contacts the heat storagetank (the surface of heat storage material), t representing an elapsedtime. Since k and A can usually be regarded as constants irrespective oftime, the equation (1) may be expressed approximately as follows:

Q=kA∫ ₀ ^(t)(Tx′−T ₀)dt [J]  (2)

[0014] Some heat storage methods mainly utilize a sensible heat of theheat storage material 31 or the heat transfer medium 47 as in the casesshown in FIGS. 11 and 12, and other heat storage methods mainly utilizea transition heat of the heat storage material 51 as in the case shownin FIG. 13. Whichever heat of a sensible heat or a transition heat maybe utilized, in order to store the heat of, for example, a temperatureof T_(x), it is necessary for the surface temperature T_(x)′ of the heatstorage material to be maintained at a temperature of at least T_(x).However, the greater is the temperature difference between the surfacetemperature T_(x)′ of the heat storage material and the ambienttemperature T₀ in the outside, and also the longer is the storage time,the larger the heat loss represented in the equation (2) becomes, whichresults in a marked reduction in heat storage efficiency.

[0015] In order to reduce the heat loss from the heat storage tank inthe equation (2), therefore, it is necessary to reduce the overall heattransfer coefficient k and/or the surface area A and shorten the time t,or to reduce the difference between the surface temperature of the heatstorage material T_(x)′ and the environmental temperature T₀.

[0016] As a method for reducing the overall heat transfer coefficient k,an attempt to install a heat insulator around the periphery of the heatstorage tank has been made. As a method for reducing the surface area,there has been made an attempt to make the heat storage tank have ashape having a small surface area per a unit volume, such as a cube or asphere.

[0017] As a method for shorten the time t, there has been made anattempt to optimize the system control over heat application systemafter the re-extraction of heat. However, since the time term andtemperature term in the equation (2) are associated with the originalpurpose of the heat storage, they can not be widely changed by nature.

[0018] On the other hand, in the examples shown in FIGS. 11 and 12,since the temperature of the heat storage material gradually approachesthat of the external environment from the portions closer to the wallsurfaces of the heat storage tanks 32 and 42, a temperature differenceis generated within the heat storage material. In general, since amatter expands or contracts as it changes in temperature, once atemperature difference is generated in the heat storage material, adensity difference occurs therein, and a gravity difference is caused.As a consequence, a movement called a natural convection is inducedwithin the heat storage material.

[0019] Therefore, a natural convection takes place from the vicinity ofthe wall surfaces of the heat storage tank 32 and 42 that have thehighest temperature difference between the same and the externalenvironment, that is, from the outer portions in the heat storagematerial, and it spreads throughout the heat storage material.Consequently, heat storage material having a temperature closer to thetemperature of external environment rather than to the heat stored flowsfrom-the surface side into the inner portion of the heat storagematerial, and simultaneously, the heat stored in the surface of the heatstorage material transfers, whereby heat transfer always takes placebetween the heat storage material and the external environment throughthe wall surface of the heat storage tank, and hence causes a large heatloss within the heat storage material.

[0020] In the above-described heat storage device shown FIG. 13, in theconservation process of heat, the natural convection of the heat storagematerial 51 within the heat storage tank 52 is suppressed by smallvessels 56, but a natural convection is generated within the heattransfer medium 57 stagnating in the heat storage tank 52. Accordingly,with regard to the heat transfer medium 57, the same as the case of theheat storage materials 31 and 47 of the heat storage devices shown inFIGS. 11 and 12 holds true.

[0021] In this way, since the temperature of the heat storage materialchanges to the temperature close to that of the external environmentfrom the heat storage material in the portion closer to the wall surfaceof heat storage tanks, a temperature difference is generated between theouter and inner portions within the heat storage material (in the caseof spherical heat storage tank, in the radial direction) in the processof heat transfer. Consequently, since a natural convection is inducedwithin the heat storage material, heat transfer always takes placebetween the heat storage material and the external environment throughthe wall surface of heat storage tank, incurring a heat loss of the heatstorage material.

[0022] In the above-described conventional heat storage devices shown inFIGS. 11 through 13, since the heat storage materials tend to easilyflow in any case, a natural convection throughout the heat storagematerial is prone to occur not only in the process of conservation ofheat, but also particularly in the processes of the injection andextraction of heat due to a temperature difference as a result of heatexchange. This incurs an increase in the overall heat transfercoefficient k in the equation (2), and an increase in the heat loss fromthe heat storage material to the outside, which constitutes one of thefactors reducing a heat recovery rate.

SUMMARY OF THE INVENTION

[0023] The present invention has been achieved to overcome the heat lossfrom a heat storage material as described above and aims to provide aheat storage device capable of reducing the influence of externalenvironment on the inside portion of a heat storage tank and suppressingthe heat loss toward the outside, by reducing the temperature differencebetween the surface of the heat storage material and the externalenvironment, and at the same time, by suppressing the heat transferbetween the surface of the heat storage material and the inside portionof the heat storage material which executes injection and extractionbetween the same and the external environment.

[0024] A first heat storage device for achieve the above-mentionedobject comprises: a heat storage tank charged with a heat storagematerial for storing the heat provided from the outside; and heatexchange means which executes the injection and extraction of heatbetween the inside of a heat storage tank and the outside by the heatexchange between the heat storage material and the heat transfer medium,wherein the heat exchange means is disposed so as to execute a heatexchange between the central portion of the heat storage tank and theoutside, or so that the central portion and the outer portion in theheat storage tank being caused to perform a heat exchange with theoutside sequentially or individually, and wherein suppressing means aredisposed in the outer portion in the heat storage tank, for suppressingthe natural convection of the heat storage material.

[0025] A second heat storage device for achieve the above-mentionedobject comprises: a heat storage tank charged with a heat transfermedium which also serves as a heat storage material for storing heatsupplied from the outside; and heat transport means which executes aninjection and extraction of heat between the inside of the heat storagetank and the outside by the inflow and outflow of the heat transfermedium, wherein the heat transport means is disposed so as to execute aheat transport between the central portion of the heat storage tank andthe outside, and wherein suppressing means are disposed in the outerportion in the heat storage tank, for suppressing the natural convectionof the heat storage material.

[0026] In the above-described heat storage devices, suppressing meansmay be constituted by: dispersing a liquid-absorbent material into theheat storage material or heat transfer medium in the outer portion inthe heat storage tank; providing the heat storage material or heattransfer medium in the outer portion in the heat storage tank with aproperty of increasing viscosity by the application of voltage, andproviding the heat storage device with means for applying power betweena pair of electrodes which are disposed on opposite sides of the outerportion in the heat storage tank so as to sandwich the outer portion; orproviding the heat storage material or heat transfer medium in the outerportion in the heat storage tank with a property of increasing viscosityby the application of magnetic force, and providing the heat storagedevice with a magnet for exerting a magnetic force on the outer portionin the heat storage tank; and further providing the outer portion in theheat storage tank with a barrier for hindering the natural convection ofthe heat storage material or heat transfer medium.

[0027] Also, in the above-described heat storage devices, there can beprovided means for promoting the heat transfer between the centralportion and the outer portion in the heat storage tank.

[0028] In the heat storage device having the above-describedconstitution, the heat of a temperature more than necessary at the heatextraction is injected to the heat storage tank by heat exchange meanswhich execute a heat exchange with the central portion in the heatstorage tank or by heat transport means which transport heat to thecentral portion, and then the heat is transferred to, stored in, andconserved in the heat storage material or heat transfer medium which isaccommodated mainly in the outer portion in the heat storage tank. Asnecessary, the heat stored is extracted from the central portion of heatstorage tank.

[0029] When injecting or extracting heat, since a heat exchange isexecuted mainly in the central portion in heat storage tank whereconvection easily occurs, a large heat transfer due to naturalconvection heat transfer joins in the heat transfer due to a heatconduction, and thereby an efficient heat exchange with the outside isachieved.

[0030] When conserving heat, since a heat transfer is usually generatedbetween the outer portion in the heat storage tank and the externalenvironment, a temperature difference due to heat transfer is generatedin the heat storage tank, and consequently, a natural convection triesto arise from the heat storage material or heat transfer medium locatedcloser to the heat storage tank. However, in the above-described heatstorage devices, the suppressing means for suppressing a naturalconvection, disposed in the outer portion in the heat storage tank, makeit difficult for the outer portion to perform a large heat transfer dueto natural convection.

[0031] As described above, in the present invention, the naturalconvection is suppressed in the outer portion in the heat storage tank,and the natural convection generated on the surface of the heat storagematerial in the outer portion in the heat storage tank is hindered fromextending throughout the heat storage material in the outer portion inthe heat storage tank, and consequently, the heat transfer and overallheat transfer coefficient (k) due to the convection between the outerportion in the heat storage tank and the external environment isreduced, and at the same time, the heat within the heat storage materialin the outer portion in the heat storage tank is hindered fromtransferring to the surface of the heat storage material. This creates agreat temperature gradient within the heat storage material in the outerportion, and gradually reduces the temperature gradient between thesurface of heat storage material and the external environment.

[0032] It is therefore possible to reduce the heat loss from the insideof the heat storage tank to the external environment while maintainingthe temperature of the inside of the heat storage tank substantiallyconstant.

[0033] In the present invention, the suppressing means against naturalconvection can be constituted by dispersing a liquid-absorbent materialinto the heat storage material or heat transfer medium in the outerportion in the heat storage tank. In this case, the above-mentionedliquid-absorbent material stagnates in the form of three-dimensionalmeshes in the outer portion in the heat storage tank, by adsorbing apart of the heat storage material or heat transfer medium and expanding.That is, since the heat storage material or the like comes to fill gapswithin the liquid-absorbent material so as to be difficult to move, thenatural convection of the heat storage material in the outer portion issuppressed, which enables the reduction in the heat loss from the insideof the heat storage tank to the outside.

[0034] As the above-mentioned suppressing means, when providing the heatstorage material or heat transfer medium in the outer portion in theheat storage tank with a property of increasing viscosity by theapplication of voltage, and means for applying power between the pair ofelectrodes which are disposed on opposite sides of the outer portion inthe heat storage tank so as to sandwich the outer portion, an electricfield is generated in the outer portion by the application of voltage,and thereby the viscosity of the heat storage material is increased.This hinders a free movement of the heat storage material, and allowsthe suppression of the natural convection of the outer portion. Also, assuppressing means, when providing the heat storage material or heattransfer medium in the outer portion in the heat storage tank with aproperty of increasing viscosity by the application of a magnetic force,and providing a magnet for exerting a magnetic power on the outerportion in the heat storage tank, the viscosity of the heat storagematerial or the heat transfer medium is increased by the action of themagnetic field of the magnet. This hinders a free movement of the heatstorage material, and allows the suppression of natural convection ofthe outer portion.

[0035] Further, the suppressing means can be constituted as a barrierdisposed in the outer portion in the heat storage tank, for hinderingthe natural convection of the heat storage material or the heat transfermedium. In this case, since the movement of the heat storage material orheat transfer medium can physically be hindered by the barrier,appropriately disposing the barrier permits the moving speed of the heatstorage material to be significantly lower than the case withouthindrance, which results in the suppression of natural convection.

[0036] In addition, in the present invention, means for promoting heattransfer is provided between the central portion and the outer portionin the heat storage tank. When, for example, a rapid injection to and anextraction from the heat storage tank is required, it is possible tospeedily execute a heat transfer between the central portion and theouter portion where the heat transfer from the central portion isdifficult due to the suppression means against natural convection.

[0037] The heat exchange means in the present invention can be disposedso that the central portion and the outer portion in the heat storagetank are caused to sequentially perform a heat exchange with theoutside. By this constitution, when injecting heat, by utilizing theresidual heat remaining after the heating or cooling of the centralportion in the heat storage tank, it is possible to additionally heat orcool the outer portion. On the contrary, when extracting heat, after thepreheating or precooling of the heat exchanger in the outer portion inthe heat storage tank, it is possible to additionally heat or cool theheat exchanger in the outer portion. This allows the exchange of heatbetween the heat storage device and the external environment to beexecuted rapidly and efficiently.

[0038] Moreover, the heat exchange means can be disposed so that thecentral portion and the outer portion in the heat storage tank arecaused to individually perform a heat exchange with the outside. By thisconstitution, when injecting heat, a heat exchange can be executed inthe central portion or the outer portion whichever is more suitable, inaccordance with the temperature of the heat injected from the outside tothe heat storage tank. On the contrary, when extracting heat, a heatexchange may be executed in the central portion or the outer portionwhichever is more suitable, in accordance with the temperature of theheat required by the outside. Consequently, the exchange of heat betweenthe heat storage device and the external environment can be executedrapidly and efficiently.

[0039] In accordance with the above-described heat storage devicesassociated with present invention, since the ease of convection of theheat storage material or heat transfer medium within the heat storagetank varies depending on position, the injection operation from theoutside to the heat storage material and the a extraction operationsfrom the heat storage material to the outside are executed toward thecentral portion where a natural convection is easy to occur, while thestorage of heat is executed in the central portion where a naturalconvection is difficult to occur. Therefore, in the processes of theinjection and extraction of heat as well as in the process ofconservation, the natural convection occurring within the inside of theheat storage material due to the temperature difference from the outsidetakes place in the central portion in the heat storage tank and hardlytakes place in the outer portion, with the result that the overall heattransfer coefficient k becomes less than conventional cases. Thisenables the provision of a heat storage device having a low heat losstoward the outside.

[0040] Also, providing the central portion and the outer portion withheat transfer elements permits the heat transfer between the centralportion and the outer portion where heat transfer does not much occurbasically to be temporarily promoted, whereby it is possible to providea heat storage device having a low heat loss as described above whileexecuting an efficient operating of the heat storage tank in accordancewith the supply and demand of heat.

[0041] Further, disposing heat exchangers so as to make a circuitthrough the central portion and the outer portion in the heat storagetank, permits the heat exchanger to execute a direct heat exchange withthe outer portion where heat transfer does not much occur basically, andto execute an injection or an extraction of heat between the centralportion and the outer portion sequentially, whereby it is possible toprovide a heat storage device having a low heat loss as described abovewhile executing the exchange of heat between the heat storage device andthe outside rapidly and on a large scale. Also, individually disposingheat exchangers in the central portion and the outer portion enables aselective use of either the central portion or the outer portion eachhaving a different temperature, or both of them, whereby it becomespossible to provide a heat storage device having a low heat loss whileefficiently executing an heat change between the heat storage device andthe outside in accordance with the supply and demand of heat.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIGS. 1 through 10 are sectional views showing varied embodimentsof heat storage devices of the present invention.

[0043]FIG. 11 through 13 are sectional views showing varied examples ofconventional heat storage devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

[0044]FIG. 1 is an embodiment of sectional view of the heat storagedevice in accordance with the present invention. This heat storagedevice accommodates a heat storage material 1 for storing heat in a heatstorage tank 2 and has a heat exchanger 3 in a heat storage tank 2, asmeans for executing a heat exchange with the outside. As a heat storagematerial 1, various materials may be used in accordance with a requiredheat storage temperature. The heat storage material 1 may be a materialwhich makes a phase change in the processes of injection and extractionof heat. The inside space of the heat storage tank is divided by a net 4into a central portion 2 a and an outer portion 2 b surrounding thecentral portion 2. That is, within the heat storage tank 2, the net 4 isstretched throughout the boundary between the central portion 2 a andthe outer portion 2 b, and the central portion 2 a is surrounded by theouter portion 2 b via the net 4. A liquid-absorbent material 5 isdispersed only in the heat storage material 1 existing in the outerportion.

[0045] It is preferable that the above-mentioned net 4 has a stabilitytoward the heat storage material 1 and the liquid-absorbent material 5,and prevents the liquid-absorbent material 5 from mixing into thecentral portion 2 a.

[0046] The heat exchanger 3 is provided as means for executing a heatexchange between the outside of the heat storage device and the heatstorage material 1 existing in the central portion 2 a, and its materialand structure and the like are not limited as long as it is stabletoward the heat storage material 1. Preferably, the liquid-absorbentmaterial 5 is stable chemically and thermally toward the heat storagetank 2 and the net 4, and increases the viscosity of the heat storagematerial 1 existing in the outer portion 2 b, whereby it can constitutesuppressing means against natural convection. For such materials,various materials, for example, polymer material particles with a highwater-absorbency such as starch-acrylonitrile graft polymerichydrolysate, and fiber such as absorbent cotton are applicable.

[0047] Next, a description is given of the operation of the heat storagedevice shown in FIG. 1, for the case where a higher temperature thanthat of environment outside the heat storage device is stored.

[0048] In the injection process of heat, out of the heat storagematerial 1 charged into the heat storage tank 2, the one existing in thecentral portion 2 a is heated to raise temperature, using the heatexchanger 3. Once the temperature of the heat storage material 1 of thecentral portion 2 a is raised, the heat transfers from the centralportion 2 a to the outer portion 2 b, and the heat storage material 1existing in the outer portion 2 b is gradually heated.

[0049] In this case, also in the injection and extraction processes ofheat, temperature transfers from the outer portion 2 b to the externalenvironment surrounding the heat storage tank 2 through the wall of theheat storage tank 2. However, for the sake of simplicity, the heat losstoward the outside is described only in respect of the conservationprocess. In the conservation process of heat, since heat transfers fromthe central portion 2 a to the external environment surrounding the heatstorage tank 2, the temperature of the heat storage material 1 graduallydecreases. As described above, the temperature of the heat storagematerial 1 changes from the outer portion closer to the heat storagetank 2, and hence, in the heat transfer process, a temperaturedifference is generated in the direction from the central portion to theouter portion within the heat storage material 1. In this case, if theheat storage material 1 is of a free fluidity, a natural convection willbe induced within the heat storage material 1.

[0050] It is well known that the strength of this natural convection iscontrolled by the Grashof number Gr defined by the following equation.$\begin{matrix}{{G\quad r} = \frac{L^{3}g\quad \beta \quad \Delta \quad T}{v^{2}}} & (3)\end{matrix}$

[0051] L: representative length(e.g., the height of vertical wallsurface of the heat storage tank 2).

[0052] g: acceleration of gravity.

[0053] β: coefficient of volume expansion of the heat storage material1.

[0054] ΔT: temperature difference generated within the heat storagematerial 1.

[0055] ν: coefficient of kinematic viscosity of the heat storagematerial 1.

[0056] If there is no mass transfer within the heat storage material 1,the heat transfer between the heat storage material 1 and heat storagetank 2 will depend only on the direct heat transfer between componentmolecules/atoms which are adjacent to each other in the heat storagematerial 1 and the heat storage tank 2, that is, only on heatconduction. However, if there is an above-described natural convectionwithin the heat storage material 1, there will exist, in addition toheat conduction, a fast and large heat transfer (heat transfer) due tonatural convection, or mass transfer, and thus a free and active heattransfer will occur. It is well known that the strength of the heattransfer between a fluid and a solid wall surface is controlled by theNusselt number Nu defined by the following equation. $\begin{matrix}{{N\quad u} = {\frac{h\quad x}{\lambda} = {C\left( {P\quad r\quad G\quad r} \right)}^{m}}} & (4)\end{matrix}$

[0057] h: coefficient of heat transfer between the heat storage material1 and the heat storage tank 2.

[0058] x: distance from the bottom of the vertical wall surface of theheat storage tank 2.

[0059] λ: coefficient of heat conduction of the heat storage material 1.

[0060] Pr: Prandtl number of the heat storage material 1 (Pr=ν/α, whereα denotes the thermal diffusivity of the heat storage material 1).

[0061] C, m: constants determined by the state of a wall surface.

[0062] The right side of the equation (4) shows that Nusselt number Nuis a function of Prandtl number Pr and Grashof number. From theequations (3) and (4), it is seen that the coefficient of heat transferbetween the heat storage material 1 and the heat storage tank 2 isexpressed by: $\begin{matrix}{h = {C\frac{\lambda}{x}\left( \frac{L^{3}g\quad \beta \quad \Delta \quad T}{\alpha \quad v} \right)^{m}}} & (5)\end{matrix}$

[0063] The equation (5) shows that, the higher the coefficient of heatconduction, the coefficient of volume expansion of the heat storagematerial 1, the vertical length of the heat storage tank 2, or thetemperature difference between the heat storage material 1 and the heatstorage tank 2, the higher the magnitude of the coefficient ofheat-transfer h, that is, the heat transfer between the heat storagematerial 1 and the heat storage tank 2 becomes, and that, on thecontrary, the higher the thermal diffusivity, or the coefficient ofkinematic viscosity of the heat storage material 1, the lower thetransfer between the heat storage material 1 and the heat storage tank 2becomes.

[0064] The quantity of heat transfer from the heat storage material 1 tothe outside follows the equation (2), and the higher the coefficient ofheat transfer h between the heat storage material 1 and the heat storagetank 2, the higher the heat loss from the heat storage material 1 to theexternal environment becomes, because the overall heat transfercoefficient k becomes higher responding the sensitively to such anincrease in the coefficient of heat transfer h.

[0065] In the heat storage device of this embodiment, since the heatstorage material 1 existing in the outer portion 2 b undergoes an actionthat is equal to the action of increasing viscosity v from theliquid-absorbent material 5 dispersed in the outer portion 2 b, anatural convection becomes difficult to arise as suggested by equation(3), and as a consequence, the heat transfer between the heat storagematerial 1 existing in the outer portion 2 b and the heat storage tank 2decreases as suggested by the equation (5), leading to a lower heat losstoward the outside.

[0066] In the traction process of heat, it is possible to extract theheat corresponding to the temperature of the heat storage material 1,from the heat storage tank 2, using the heat exchanger 3. As thetemperature of the heat storage material 1 existing in the centralportion 2 a decreases as a result of the heat extraction, the heatstorage material 1 existing in the outer portion 2 b is graduallycooled.

Embodiment 2

[0067]FIG. 2 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. This heatstorage device accommodates a heat transfer medium 8 which also servesas a heat storage material, in a heat storage tank 2. However, unlikethe case of FIG. 1, this heat storage device makes communicate thecentral portion 2 a surrounded by a net 4 in the heat storage tank 2,with the outside of the heat storage device by two communicating tubes 6a and 6 b which execute the inflow and outflow of the transfer medium 8.The other constructions are similar to the embodiment shown in FIG. 1.

[0068] Next, a description is presented of the operation of the heatstorage device in the case where a higher temperature than that of theexternal environment is stored in the heat storage device shown in FIG.2.

[0069] In the injection process of heat, the heat transfer medium 8sucked out from the communicating tube 6 b is heated by a heat source atthe outside, and is circulated back to the heat storage tank 2 using thecommunicating tube 6 b. As a consequence, the temperature of the heattransfer medium 8 existing in the central portion 2 a rises, with theresult that heat is injected to the heat storage tank 2. As thetemperature of the heat transfer medium 8 in the central portion 2 arises, the heat transfer medium 8 in the outer portion 2 b is graduallyheated.

[0070] In the conservation process of heat, since heat transfers to theexternal environment surrounding the heat storage tank 2, thetemperature of the heat transfer medium 8 gradually decreases. However,as in the case of the operation in FIG. 1, a natural convection becomesdifficult to occur due to the liquid-absorbent material 5 dispersed inthe outer portion 2 b, and consequently, the heat transfer between theheat transfer medium 8 existing in the outer portion 2 b and the heatstorage tank 2 decreases, leading to a lower heat loss toward theoutside.

[0071] In the traction process of heat, the heat transfer medium 8accommodated in the heat storage tank 2 is sucked out from thecommunicating tube 6 a, and is utilized at the outside as a heat sourcehaving the temperature of the heat transfer medium 8. The heat transfermedium 8 cooled as a result of the thermal utilization is circulatedback to the heat storage tank 2 through the communicating tube 6 b. Asthe temperature of the heat transfer medium 8 existing in the centralportion 2 a decreases as a result of the heat extraction, thetemperature of the heat transfer medium 8 existing in the outer portion2 b is gradually cooled.

Embodiment 3

[0072]FIG. 3 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. In this heatstorage device, in a heat storage tank 2 having a similar constructionas the embodiment shown in FIG. 2, heat storage materials 1 a and 1 b tobe accommodated in the central portion 2 a and the outer portion 2 b areaccommodated in small vessels 7 a and 7 b, respectively. As heat storagematerials 1 a and 1 b, various materials may be used in accordance withrequired temperatures. The heat storage material may be a material whichmakes a phase change in the processes of injection and extraction.Further, the heat storage materials la and 1 b which exist respectivelyin the central portion 2 a and the outer portion 2 b in a heat storagetank 2 may be an identical material, or may be different materials.

[0073] In the drawing, though small vessels 7 a and 7 b in thisembodiment are represented by a spherical shape, the material and shapeof the small vessel are not limited, as long as the small vessels arestable toward the heat storage materials 1 a and 1 b and the heattransfer medium 8, and the dimensions of the small vessels may bedifferent between those in the central portion 2 a and those in theouter portion 2 b, or may be the same.

[0074] Next, a description is given of the operation of the heat storagedevice in the case where a higher temperature than that of environmentoutside the heat storage device is stored in the heat storage deviceshown in FIG. 3.

[0075] In the injection process of heat, as in the case of theembodiment shown in FIG. 2, the heat transfer medium 8 heated at theoutside is circulated back to the heat storage tank, and consequently,heat transfers from the heat transfer medium 8 to the heat storagematerials la, the temperature thereof rising, thereby heat beinginjected to the heat storage tank 2. As the temperature of the heatstorage material la existing in the central portion 2 a rises, the heatstorage material 8 existing in the outer portion 2 b is graduallyheated.

[0076] In the conservation process of heat, since heat transfers to theexternal environment surrounding the heat storage tank 2, thetemperature of the heat transfer medium 8 gradually decreases. However,as in the case of the operation in FIG. 1, it becomes difficult for anatural convection to occur due to the liquid-absorbent material 5dispersed in the outer portion 2 b, and consequently, the heat transferbetween the heat transfer medium 8 existing in the outer portion 2 b andthe heat storage tank 2 decreases, leading to a lower heat loss towardthe outside.

[0077] The traction process of heat is substantially the same as that ofthe embodiment shown in FIG. 2.

Embodiment 4

[0078]FIG. 4 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. Although thisheat storage device has substantially the same construction as that ofthe embodiment shown in FIG. 1, it has a thermal insulating layer 9outside a heat storage tank 2. Therefore, the description of the commonparts with the embodiment shown in FIG. 1 is omitted, and the commonreferential numerals are marked with the identical numerals in FIG. 4.

[0079] Next, a description is given of the operation of the heat storagedevice in the case where a higher temperature than that of environmentoutside the heat storage device is stored in the heat storage deviceshown in FIG. 4.

[0080] The operation in the injection process of heat is basically thesame as that of the embodiment 1.

[0081] Although the operation in the conservation process of heat isalso basically the same as that of the embodiment 1, the coefficient ofheat conduction of the thermal insulating layer 9 is lower than that ofthe heat storage tank 2, so that it is possible to reduce the overallheat transfer coefficient k shown in the equation (2) and to provide aheat storage device with a further lower heat loss.

[0082] The operation in the extraction process of heat is basically thesame as that of the embodiment 1.

[0083] In this embodiment, a case is shown where the heat storage tankand the like is same as those in the embodiment 1, but these operationsare applicable also for such constructions as shown in the embodiment 2or 3.

Embodiment 5

[0084]FIG. 5 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. Although thisheat storage device has a similar heat storage tank 2 and a heatexchanger 3 to the embodiment shown in FIG. 1, it has an electrode 4 aso as to spatially separate the central portion 2 a and the outerportion 2 b, in place of the net 4 shown in FIG. 1. Also, an electrode 4b is stretched around in the vicinity of the inner surface of the heatstorage tank 2. The electrodes 4 a and 4 b are disposed on oppositesides so as to sandwich the outer portion 2 b, each connected to a powersupply 11. Inside the heat storage tank 2, an electrorheological fluid10 is charged, which has a property of increasing viscosity byapplication of voltage and which serves as a heat storage material.

[0085] The material and structure of the electrodes 4 a and 4 b are notlimited, as long as they are capable of generating a uniform electricfield in the outer portion 2 b by the application of voltage to thepower supply 11. They may, for example, be reticular. Preferably, theelectrorheological fluid 10 is stable toward the heat storage tank 2,the heat exchanger 3, the electrodes 4 a and 4 b. For anelectrorheological fluid, various materials such as, for example, aliquid which is suspending carbon particles are applicable in accordancewith required heat storage temperatures.

[0086] Next, an explanation is presented of the operation of the heatstorage device in the case where a higher temperature than that of theexternal environment is stored in the heat storage device shown in FIG.5.

[0087] In the injection process of heat, out of the electrorheologicalfluid (heat storage material) 10 accommodated in the heat storage tank2, the one existing in the central portion 2 a is heated to raisetemperature, using the heat exchanger 3, and thereby theelectrorheological fluid 10 existing in the outer portion 2 b isgradually heated.

[0088] If the quantity of heat supplied from the outside to the heatstorage device is lower than or equal to that of the quantity of heatdiffusing from the central portion 2 a to the outer portion 2 b, then avoltage is applied between 4 a and 4 b by the power supply 11. As aconsequence, an electric field is generated between 4 a and 4 b, and theviscosity of the electrorheological fluid 10 existing in the outerportion 2 b sandwiched by the electrodes 4 a and 4 b is increased,resulting in a lower heat loss toward the outside.

[0089] If the quantity of heat supplied from the outside to the heatstorage device is larger than the quantity of heat diffusing from thecentral portion 2 a to the outer portion 2 b, then it will be possibleto reduce the viscosity of the outer portion 2 b by not applying avoltage between the electrodes 4 a and 4 b, and to allow the heattransfer from the central portion 2 a to the outer portion 2 b to freelytake place, thereby causing the heat injected from the outside to beabsorbed efficiently into the heat storage tank 2.

[0090] In the conservation process of heat, a voltage is applied to theelectrodes 4 a and 4 b by the power supply 11. The application of avoltage reduces the heat loss from the heat storage device to theoutside for the foregoing reason.

[0091] In the extraction process of heat, the heat corresponding to thetemperature of the electrorheological fluid 10 can be extracted from theheat storage tank 2 through the heat exchanger 3. As the temperature ofthe electrorheological fluid 10 in the outer portion 2 b decreases as aresult of the extraction of heat, the electrorheological fluid 10 in thecentral portion 2 a gradually cooled.

[0092] If the quantity of heat required of the heat storage device bythe outside is met by the heat conserved in the central portion 2 a, avoltage will be applied to the electrodes 4 a and 4 b by the powersupply 11. The application of a voltage reduces the heat loss from theheat storage device to the outside for the foregoing reason. If thequantity of heat required of the heat storage device by the outside isnot met by only the heat conserved in the central portion 2 a, it willbe possible to reduce the viscosity of the outer portion 2 b by notapplying a voltage between the electrodes 4 a and 4 b, and to allow theheat transfer from the outer portion 2 b to the central portion 2 a tofreely take place, thereby causing the heat needed at the outside to beextracted efficiently from the heat storage tank 2.

[0093] Further, as in the case of embodiment 4, a heat storage devicehaving a lower heat loss can be achieved by covering the heat storagetank with the thermal insulating layer 9.

Embodiment 6

[0094]FIG. 6 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. This heatstorage device has a similar heat storage tank 2 and heat exchanger 3 tothe embodiment shown in FIG. 1, but in the heat storage tank 2 amagnetic fluid 12 is charged, which has a property of increasingviscosity by the application of a magnetic field.

[0095] On the boundary between the central portion 2 a and the outerportion 2 b in the heat storage tank 2, and outside the heat storagetank 2, a multitude of electromagnets 13 a and 13 b which generate amagnetic force by a application of an external power supply, aredisposed by an arbitrary number. In the drawing, the wiring between theelectromagnets 13 a and 13 b and the power supply 11 are omitted.

[0096] The electromagnet 13 a is disposed so that the magnetic polesface away from the central portion 2 a, and on the contrary, theelectromagnet 13 b is disposed so that the magnetic poles face away fromthe outside of the heat storage device. The material and structure ofelectromagnets 13 a and 13 b are not limited as long as they cangenerate a magnetic field only in the outer portion. For example,permanent magnets may be used in place of the electromagnets 13 a and 13b. Also in that case, the basic operation is the same. For anelectrorheological fluid 12, various materials, such as a liquid whichis suspending carbon particles, are applicable in accordance withrequired heat storage temperatures.

[0097] The processes of injection, conservation and extraction of heatin the heat storage device shown in FIG. 6 are the same as those in theembodiment shown in FIG. 5, except that the viscosity of the magneticfluid 12 is increased by the generation of the magnetic field throughthe energizing of the electromagnets 13 a and 13 b in place of thegeneration of electric field.

[0098] The operation in the case where the electromagnets 13 a and 13 bare replaced with permanent magnets is the same as that of the casewhere the electromagnets 13 a and 13 b are energized. Further, as in thecase of the embodiment 4, a heat storage device with a lower heat losscan also be achieved by covering the heat storage tank with the thermalinsulating layer 9.

Embodiment 7

[0099]FIG. 7 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. This heatstorage device accommodates a heat storage material 1 for storing heat,in a heat storage tank 2 thereof, and has a heat exchanger 3 forexecuting a heat exchange with the outside, in the heat storage tank 2,as in the case of the embodiment shown in FIG. 1. However, it has amultitude of barriers 14 disposed in the outer portion 2 b partitionedby a net 4 in the a heat storage tank 2. For the barriers 14, forexample, reticular ones may be applicable.

[0100] In this embodiment, a liquid-absorbent material 5 used in theembodiment shown in FIG. 1 is not used, but it may be employed. Becauseother constructions are the same as the embodiment shown in FIG. 1,description of them is omitted.

[0101] Next, an explanation is presented of the operation of the heatstorage device in the case where a higher temperature than that ofenvironment outside the heat storage device is stored in the heatstorage device shown in FIG. 7.

[0102] The operation in the injection process of heat is basically thesame as that of the embodiment 1. In the conservation process of heat,since heat transfers from the central portion 2 a to the externalenvironment, the temperature of the heat storage material 1 graduallydecreases. However, even if the heat storage material 1 of the outerportion 2 b tries to transfer to the central portion 2 a, it willrepeatedly collide against the minutely intricated barriers 14, so thatthe moving speed of the heat storage material 1 becomes lower than thecase without the barriers 14, and thereby the heat storage material 1undergoes an action that is apparently equal to the action of increasingviscosity ν of the heat storage material 1 existing in the outer portion2 b, which leads to a lower heat loss toward the outside.

[0103] The operation in the extraction process of heat is also basicallythe same as that of the embodiment 1. Further, as in the case of theembodiment 4, a heat storage device having a lower heat loss can beachieved by covering the heat storage tank with the thermal insulatinglayer 9.

Embodiment 8

[0104]FIG. 8 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. This heatstorage device has basically the same construction as the embodimentshown in FIG. 1, and in the outer portion 2 b partitioned by a net 4 ina heat storage tank 2, a liquid-absorbent material 5 is dispersed in aheat storage material 1. Further, a heat transfer element 15 capable ofarbitrarily promoting heat transfer is disposed between the centralportion 2 a and the outer portion 2 b in the heat storage tank. The heattransfer element 15 may be, for example, one capable of transporting anyquantity of heat in any direction by the application of voltage, or maybe one which markedly varies in the heat transfer resistance withtemperature, such as a heat pipe.

[0105] Next, an explanation is presented of the operation of the heatstorage device in the case where a higher temperature than that of theexternal environment is stored in the heat storage device shown in FIG.8.

[0106] The operation in the injection process of heat is basically thesame as the case of the embodiment 1. However, if the quantity of heatsupplied by the outside is more than heats the heat storage material 1of the central portion 2 a, it is possible, in addition to the procedurein the embodiment 1, to promote the heat transfer from the centralportion 2 a, to the outer portion 2 b by the heat transfer element 15,and thereby store the heat supplied by the outside in the heat storagedevice as completely and rapidly as possible. The operation in theconservation process of heat is also similar to that of the embodiment1, and the heat loss from the heat storage device is less thanconventional cases.

[0107] The operation in the extraction process of heat is also basicallythe same as that of the embodiment 1. However, if the quantity of heatsupplied by the outside is not met by only the use of the heat stored inthe heat storage material 1 existing in the central portion 2 a, it ispossible, in addition to the procedure in the embodiment 1, to promotethe heat transfer from the outer portion 2 b to the central portion 2 aby the heat transfer element 15, and thereby to extract the heatrequired by the outside from the heat storage device as completely andrapidly as possible.

[0108] In this embodiment, a case where the heat storage tank and thelike are similar to those in the embodiment 1 is shown, but similaroperations to the foregoing can be achieved also in the case of suchconstructions as shown in the embodiments 2 through 7.

Embodiment 9

[0109]FIG. 9 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. This heatstorage device has basically the same construction as the embodimentshown in FIG. 1, and provides a heat storage tank 2 with an heatexchanger 16 disposed so as to sequentially pass the central portion 2 aand the outer portion 2 b, for executing a heat exchange between theoutside. Also, a liquid-absorbent material 5 is dispersed in the heatstorage material 1 of the outer portion 2 b partitioned by a net 4 inthe heat storage tank 2. With regard to the heat exchanger 16, theconstitutions such as material, the cross-sectional profile, and thepresence or absence of a fin is not limited, as long as the heatexchanger 16 has a stability toward the heat storage material 1.

[0110] Next, an explanation is presented of the operation of the heatstorage device in the case where a higher temperature than that of theexternal environment is stored in the heat storage device shown in FIG.9.

[0111] The operation in each process is basically the same as that ofthe embodiment 1. In the injection process of heat, firstly the centralportion 2 a of the heat storage material 1 charged into heat storagetank 2 is heated to increase temperature using the heat exchanger 16.Next, the heat storage material 1 of the outer portion 2 b is heated byusing the residual heat. In the embodiment 1, the heat storage material1 of the outer portion 2 b is gradually heated through the heat transferdue to the temperature increase of the heat storage material 1 of thecentral portion 2 a, but the outer portion 2 b has a high apparentviscosity as described in the embodiment 1, so that a heat transfer isdifficult to occur. By utilizing the heat exchanger 16, a more rapidinjection of heat than the case of the embodiment 1 can be achieved.

[0112] The conservation process of heat is similar to that of theembodiment 1.

[0113] In the extraction process of heat, by using the heat existing inthe outer portion 2 b out of the heat storage material 1 charged intothe heat storage tank 2, the heat transfer medium to perform a heatexchange with the outside are heated by way of preheating. Next, theheat transfer medium is heated by the heat of the heat storage material1 of the central portion 2 a. In this embodiment, as in the case of theembodiment 1, the outer portion 2 b is high in apparent viscosity so asto be difficult to perform heat transfer. However, use of the heatexchanger enables a more rapid injection of heat than the case of theembodiment 1.

[0114] In this embodiment, a case where the heat storage tank and thelike are similar to those in the embodiment 1 is shown, but similaroperations to the forgoing can be achieved also when applied to suchstructures as shown in the embodiments 2 through 8.

Embodiment 10

[0115]FIG. 10 is a sectional view of another embodiment of the heatstorage device in accordance with the present invention. This heatstorage device has heat exchangers 17 a and 17 b capable of heating orcooling the central portion 2 a and the outer portion 2 b in a heatstorage tank 2 individually, in place of the heat exchanger 16 in theembodiment shown in FIG. 9. Since the material of the heat exchanger 17a and 17 b are the same as those of the embodiment shown in FIG. 9 andthe other constructions are also the same as those of the embodimentshown in FIG. 9, description of constructions is omitted.

[0116] Next, an explanation is presented of the operation of the heatstorage device in the case where a higher temperature than that of theexternal environment is stored in the heat storage device shown in FIG.10.

[0117] The operation in each process is basically the same as that ofthe embodiment 1. In the injection process of heat, procedure varies inaccordance with the following three cases.

[0118] A first procedure is adopted when the temperature of the heatsource is higher than that of the central portion 2 a of the heatstorage material 1, and at the same time, when the quantity of the heatof the heat source is more than heats the central portion 2 a. At thistime, the heat transfer medium which exchanges heat with the outside isfirst sent from the heat source to the heat exchanger 17 a, and heatsthe heat storage material 1 of the central portion 2 a, being thenreturned to the outside. However, since the temperature of this heattransfer medium returned is higher than that of the heat storagematerial 1 of the outer portion 2 b, the heat transfer medium is nextsent to the heat exchanger 17 b, and heats the heat storage material 1of the outer portion 2 b, being then returned to the outside. This makesit possible to rapidly heat the heat storage material 1 of the outerportion 2 b and to improve the efficiency of heat exchange in the heatsource. This is because the temperature difference at heat exchange inthe heat source becomes large as compared with the case where only theheat storage material 1 of the central portion 2 a is heated, since theheat transfer medium sent from the heat source is returned thereto withthe temperature decreased.

[0119] A second procedure in the injection process of heat is adoptedwhen the temperature of the heat source is higher than that of thecentral portion 2 a of the heat storage material 1, and at the sametime, when the quantity of the heat of the heat source is all that isneeded to heat the central portion 2 a. At this time, the heat transfermedium from the heat source is sent to the heat exchanger 17 a, andheats the heat storage material 1 of the central portion 2 a, being thenreturned to the heat source. Although the temperature of this heattransfer medium returned from the 17 a is higher than that of the heatstorage material 1 of the outer portion 2 b, further heat transfermedium can not be sent to the heat exchanger 17 b, unlike the case ofthe first procedure. This is because the quantity of heat of the heatsource in this case is not sufficient, and hence the heat transfermedium which has further decreased in temperature as a result of usageof the heat exchanger can not be heated by the heat source to a highertemperature than that of the heat storage material 1 of the centralportion 2 a, with the result that a circulation wherein heat starts fromthe heat source and returns thereto via the heat exchangers 17 a and 17b can not thermally exist.

[0120] A third procedure in the injection process of heat is adoptedwhen the temperature of the heat source is lower than that of thecentral portion 2 a of the heat storage material 1, and at the sametime, when it is higher than that of the heat storage material 1 of theouter portion 2 b. In this case, the heat transfer medium is sent to theheat exchanger 17 a, and heats the heat storage material 1 of the outerportion 2 b, being then returned to the heat source. Since thetemperature of the outer portion 2 b lies between the temperature of thecentral portion 2 a and the that of environment outside the heat storagedevice, it is possible to cause the heat storage device to efficientlystore the energy of the heat source by using the heat exchanger 17 balone, even if the temperature of the heat source is not sufficientlyhigh.

[0121] In the conservation process of heat, as in the case of theembodiment 1, the apparent viscosity of the heat storage material 1 ofthe outer portion 2 b is high so that the heat loss toward the outsideis low.

[0122] Also in the extraction process of heat, the procedure varies inaccordance with the following three cases.

[0123] A first procedure is adopted when the temperature of the heatneeded at the outside is close to that of the heat storage material 1 ofthe central portion 2 a, and at the same time, when it can not be met byonly the heat of the central portion 2 a. At this time, the heattransfer medium exchanging heat with the outside is first sent from theheat source to the heat exchanger 17 b, and is preheated by the heatstorage material 1 of the outer portion 2 b, being then once returned tothe outside. Since the temperature of the heat transfer medium returnedfrom the heat exchanger 17 b is lower than that of the heat storagematerial 1 of the central portion 2 a, the heat transfer medium is nextsent to the heat exchanger 17 a, and is heated by the heat storagematerial 1 of the central portion 2 a, being then returned to theoutside. This makes it possible to rapidly extract the heat of the heatstorage material 1 of the outer portion 2 b and to suppress a rapidtemperature decrease of the central portion 2 a caused by a large loadfrom the outside.

[0124] A second procedure in the extraction process of heat is adoptedwhen the temperature of the heat needed at the outside is close to thatof the heat storage material 1 of the central portion 2 a, and at thesame time, when it can be met by the quantity of heat extracted from thecentral portion 2 a. At this time, the heat transfer medium from theoutside is sent to the heat exchanger 17 a, and is heated by the heatstorage material 1 of the central portion 2 a, being then returned tothe outside for utilizing.

[0125] A third procedure in the extraction process of heat is adoptedwhen the temperature of the heat needed at the outside is lower thanthat of the heat storage material 1 of the central portion 2 a, but atthe same time, when it is higher than that of the heat storage material1 of the outer portion 2 b. In this case, the heat transfer medium fromthe outside is heated by the heat exchanger 17 b, and then it isreturned to the outside for utilizing. If the whole of the heat transfermedium is sent from the outside to the heat exchanger 17 a, thetemperature of the heat storage material 1 of the central portion 2 ahaving a higher temperature than the outer portion 2 b is reduced, whichcauses a decrease in the temperature of the heat storage material 1existing in the central portion 2 a having a temperature higher thanthat of the outer portion 2 b. This is undesirable because the effectiveenergy (exergy) of the heat storage device is rendered less, even if thequantity of heat to be extracted from the heat storage device to theoutside is the same.

[0126] A third procedure is to prevent the reduction in theeffectiveness of heat storage device as much as possible, by extractingthe heat from a suitable portion of the heat storage material 1corresponding to the temperature of the heat required by the outside.This makes it possible to provide a heat storage device that efficientlyoperates in spite of a wide variation in supply and demand of heat. Inthis embodiment, a case where the heat storage tank and the like aresimilar to those in the embodiment 1 is shown, but similar operations tothe foregoing can be achieved also in the case of such constructions asshown in the embodiments 2 through 8.

[0127] Incidentally, when the heat of a temperature lower than that ofthe environment outside heat storage device is stored in the storagedevice of each embodiment, only the high-lower relation of temperaturebetween the storage device and the external environment, and thedirection of heat flow become opposite to those of each of theabove-described embodiments. However, the basic operation is the same asa corresponding embodiment.

[0128] The shape of the heat storage tank 2 in these embodiments are notlimited to the cuboid shape as shown in the drawings. Various shapessuch as cylindrical, or spherical shape may also be adopted.

1. A heat storage device comprising: a heat storage tank charged with aheat storage material for storing the heat supplied from the outside;heat exchange means for executing an injection and an extraction of heatbetween the inside of the heat storage tank and the outside by the heatexchange between said heat storage material and a heat transfer medium;said heat exchange means being disposed so as to execute a heat exchangebetween the central portion in said heat storage tank and the outside;and suppressing means for suppressing the natural convection of the heatstorage material, said suppressing means being disposed in the outerportion in the heat storage tank.
 2. A heat storage device comprising: aheat storage tank charged with a heat storage material for storing theheat supplied from the outside; heat exchange means for executing aninjection and an extraction of heat between the inside of the heatstorage tank and the outside by the heat exchange between said heatstorage material and a heat transfer medium; said heat exchange meansbeing disposed so that the central portion and the outer portion in saidheat storage tank are caused to sequentially perform a heat exchangewith the outside; and suppressing means for suppressing the naturalconvection of the heat storage material, said suppressing means beingdisposed in the outer portion in the heat storage tank.
 3. A heatstorage device comprising: a heat storage tank charged with a heatstorage material for storing the heat supplied from the outside; heatexchange means for executing an injection and an extraction of heatbetween the inside of the heat storage tank and the outside by the heatexchange between said heat storage material and a heat transfer medium;said heat exchange means being disposed so that the central portion andthe outer portion in said heat storage tank are caused to individuallyperform a heat exchange with the outside; and suppressing means forsuppressing the natural convection of the heat storage material, saidsuppressing means being disposed in the outer portion in the heatstorage tank.
 4. A heat storage device comprising: a heat storage tankcharged with a heat transfer medium which also serves as a heat storagematerial for storing the heat supplied from the outside; heat transportmeans which execute an injection and an extraction of heat between theinside of the heat storage tank and the outside by the inflow and theoutflow of said heat transfer medium; said heat transport means beingdisposed so as to execute a heat transport between the central portionin said heat storage tank and the outside; and suppressing means forsuppressing the natural convection of the heat storage material, saidsuppressing means being disposed in the outer portion in the heatstorage tank.
 5. A heat storage device as claimed in any one of claims 1through 4, wherein suppressing means is constituted by dispersing aliquid-absorbent material in the heat storage material or the heattransfer medium in the outer portion in the heat storage tank.
 6. A heatstorage device as claimed in any one of claims 1 through 4, wherein: assuppressing means, a property of increasing viscosity by the applicationof voltage is provided for the heat storage material or the heattransfer medium in the outer portion in the heat storage tank; and meansfor applying power between a pair of electrodes is further provided, assuppressing means, on opposite sides of the outer portion in the heatstorage tank so as to sandwich said outer portion.
 7. A heat storagedevice as claimed in any one of claims 1 through 4, wherein: assuppressing means, a property of increasing viscosity by the applicationof a magnetic force is provided for the heat storage material or theheat transfer medium in the outer portion in the heat storage tank; anda magnet for exerting the magnetic force is further provided, assuppressing means, in the outer portion in the heat storage tank.
 8. Aheat storage device as claimed in any one of claims 1 through 4, furthercomprising: a barrier, as suppressing means, for hindering the naturalconvection of the heat storage material or the heat transfer medium,said barrier being disposed in the outer portion in the heat storagetank.
 9. A heat storage device as claimed in any one of claims 1 through4, further comprising: means for promoting a heat transfer, said meansbeing disposed between the central portion and the outer portion in theheat storage tank.